Is zakat due on a rented apartment, and is it calculated based on market value or actual value, given the existence of debt and an expenditure for rent before the Hawl (one year)?

Zakat (definition) is not obligatory on an apartment designated for rent. Rather, it becomes obligatory on the income generated from its rent if it reaches the nisab (definition) (minimum threshold) and a full hawl (lunar year) passes over it. So, if the rent money is received, and debts are deducted from it, and what remains reaches the nisab and a full hawl passes over it without decreasing during the hawl, then zakat becomes obligatory on it, which is one-quarter of one-tenth (2.5%). However, if the money is spent before a full hawl passes over it, then there is no zakat on it.